Dielectric liquid and apparatus utilizing same



C. D. COOK March 28, 1961 DIELECTRIC LIQUID AND APPARATUS UTILIZING SAME Filed Jan. 20. 1954 M Q hi 4 mm H United States Patent DIELECTRIC LIQUID AND APPARATUS UTILIZING SAME Clinton D. Cook, Burlington, Vt., assignor to General Electric Company, a corporation of New York Filed Jan. 20, 1954, Ser. No. 405,103

9 Claims. (Cl. 336-58) This invention relates to an improved dielectric liquid; more particularly, the invention relates to a liquid-cooled transformer utilizing an improved liquid dielectric material as the coolant.

The cooling of transformers by means of a liquid dielectric material has been a practice of long standing. Liquids such as refined petroleum oils and halogenated aromatic compounds have been used as transformer cooling agents. In general, it is desirable that liquids utilized to effect transformer cooling have good corona resistance, high impulse strength, low power factor, high dielectric strength, and stability to oxidation. No single transformer coolant is superior in all respects to all others. Consequently, in choosing a particular cooling liquid for a particular installation a number of factors must be given consideration.

It is an object of this invention to provide a liquidcooled transformer with improved electrical properties.

It is another object of this invention to provide a liquidcooled transformer utilizing as the coolant a liquid oflering great resistance to oxidation.

It is another object of the invention to provide a dielectric fluid suitable for use as a transformer cooling agent.

It is a further object of the invention to provide a transformer having incorporated therein as a cooling fluid a tertiary alkyl derivative of naphthalene.

The above and other objects will be apparent from the following specification considered in connection with the accompanying drawing which is a broken sectional view of a transformer constructed in accordance with this invention.

Briefly stated, in accordance with one of its aspects, this invention comprises a transformer having a core piece, a plurality of windings on the core piece, a liquid-tight housing encasing the windings and core piece, and a liquid in the housing surrounding the windings and core piece, the liquid consisting essentially of a tertiary alkyl naphthalene wherein the alkyl group contains from 4 to 8 carbon atoms.

The transformer consists of a liquid-tight housing or tank 3 fitted with a removable cover 7 provided with a pair of external terminals 4 and 5. Extending vertically with respect to the tank 3 are a plurality of cooling conduits 9 through which cooling fluid circulates. Positioned on the interior of the tank 3 is a core piece 1 and a plurality of electrical windings 2 in accordance with conventional transformer construction. The core piece 1 and windings 2 are immersed in a dielectric liquid 6 which will be described in more detail hereinafter.

In accordance with this invention, the dielectric fluid 6 is a monotertiary alkyl naphthalene or a monotertiary alkyl methyl naphthalene wherein the tertiary alkyl group contains from 4 to 8 carbon atoms. While monotertiary butyl naphthalene is the preferred dielectric liquid, the other liquids in the class defined above are also satisfactory. The naphthalene derivatives of this invention are not new to the art. They may be prepared by reacting naphthalene or methyl naphthalene with the appropriate tertiary alkyl alcohol in the presence of aluminum chloride according to the method of Trukervank and Vichrova [1. Gen. Chem. (USSR) 7, 632-6 (1937); CA. 30, 442]. Instead of using a tertiary alkyl alcohol in the reaction the corresponding alkene may be substituted. Tertiary alkyl halides may also be substituted for the alcohols. In running the reaction care must be exercised to avoid the formation of polyalkyl derivatives. The preparation of the liquid 6 by both the alkyl halide and alkene process will be described below.

EXAMPLE I Preparation of tertiary butyl naphthalene Naphthalene (10 pounds) and di-tertiary butyl nap-hthalene (7 pounds) were placed in a flask and heated to 88 C. Anhydrous aluminum chloride (30 grams, 1.06 oz.) was added to the mixture. Isobutylene was passed through the molten mixture until a total of 3 lbs., 5 oz. had been added. The reaction was carried out at a controlled temperature of to C. Vigorous stirring was employed throughout the entire period.

The residual aluminum chloride was hydrolyzed by the addition of 100 ml. of water. Soda lime (20 grams) was added to the mixture which was then given a light treatment with dry fullers earth, filtered, and distilled at atmospheric pressure. Yields were as follows:

Unreacted naphthalene, 3 lbs., 216 C.270 C.

Tertiary butyl naphthalene, 9 lbs., 1 02., 270 C.-290 C.

Di-tertiary-butyl naphthalene, 7 lbs., 11 02., 290' C.-

Material started with, 20 lbs. and 5 oz.

Material recovered, 19 lbs. and 13 oz.

Losses, 8 oz. probably handling losses and errors in weigh- EXAMPLE 2 Preparation of tertiary octyl a methyl naphthalene Alpha methyl naphthalene (1136 grams) and di-isobutylene (896 grams) were heated to 122 C. Anhydrous aluminum chloride (36 grams) were added with vigorous stirring over a 2 /2 hour period. Temperature ranged from 122 to 174 C. during the reaction. Stirring was continued for 1 /2 hours after final addition of AlCl Reaction mixture was treated with water and fullers earth then vacuum disti led. Fraction boiling at 152 C. at 2-2% mm. Hg was collected and identified as tertiary octyl a methyl naphthalene.

EXAMPLE 3 Preparation of tertiary butyl methyl naphthalene Methyl naphthalene (mixture of alpha and beta isomers) (1500 grams) was heated to 82 C. and a small amount of anhydrous aluminum chloride was added as a catalyst after which isobutylene (1220 grams) was introduced to the mixture. Vigorous stirring was maintained during the entire reaction. A total of 60 grams of aluminum chloride was added gradually as the reaction progressed. The reaction mixture was then water washed, dried over soda lime, fullers earth treated and distilled at atmospheric pressure. The fractions boiling at 265- 292 C. were collected as product. The bulk of the material came over at 29 l-292 C.

EXAMPLE 4 Preparation of tertiary butyl beta methyl naphthalene Beta methyl naphthalene (302 grams) was heated to 5060 C. with catalytic quantities of anhydrous aluminum chloride. A total of 357 ml. of tertiary butyl chloride was added dropwise over a 1% hour period. At the completion of the reaction, the mixture was treated with a: d Water and given two fullers earth filtrations then distilled at atmospheric pressure. The fractions collected boiled at 277-294 C. Most of the product distilled between 292-294 C.

EXAMPLE Preparation of tertiary amyl naphthalene Petroleum ether (boiling point 60 l20 C.) (250 ml.) was added to naphthalene (870 grams). Aluminum chloride (0.1 gram) was added to the reaction vessel and the addition of tertiary amyl chloride was begun. Frequent small additions of AlCl were necessary to enable the reaction to continue. The temperature was maintained at 7580 C. over a 2 /2 hour period. Water was added to the reaction mixture which wasthen given a fuller's earth treatment followed by fractional distillation through a two-foot column packed with /4 glass helices. The fraction boiling between 286-308 C. was collected as product.

EXAMPLE 6 Preparation of tertiary octyl naphthalene Di-isobutylene (224 grams) and naphthalene (256 grams) were heated to 80-90 C. Anhydrous aluminum chloride (10 grams) was added in small increments over a one-hour period. Stirring was continued 4 hours, after which the mixture was cooled and extracted with dilute HCl, followed by washing with water. The reaction mixture was dried over soda lime and sodium sulfate, then treated with fullers earth, filtered and distilled. The bulk of the material distilled over at 310-330 C.

While polytertiary butyl naphthalene compounds have previously been recognized as having desirable properties in electrical apparatus, the fact that monotertiary alkyl' naphthalenes possess desirable properties has previously gone unrecognized. For instance, Larsen Patent 2,436,110 discloses an electric cable in which polytertiary alkyl naphthalenes are used as a dielectric material. Example 1 of the Larsen patent describes the preparation of octyl naphthalenes and refers to a light fraction boiling fro-m 178 to 193 C. at 3 millimeters mercury pressure and a heavier fraction boiling in the range from 193 to 205 C. at 3 millimeters mercury pressure. Larsen uses the heavier fraction as his impregnant. In the present invention it is the portion corresponding to one of Larsens light fractions, which Larsen describes as having a fairly low dielectric strength which is utilized. The material of the present invention has a boiling range of l14-120 C. at 3 mm. Hg pressure and it is characterized by a dielectric strength in excess of 53 kv.

In order to promote as high a yield as possible of the monoa'lltyl compound, I prefer to have a substantial molar excess of naphthalene present in the reaction mixture. Temperatures over 125 C. promote the production of tarry high boiling residues. Temperatures below 75 C. promote the polymerization of the tertiary alkyl compound when alkenes are used in the reaction.

Vhile aluminum choride has been the best catayst used in the reaction, aiurninum bromide, ferric chloride, and other Friedel-Crafts catalysts'are satisfactory. The product produced in accordance with this invention usually consists of an isomeric mixture in which the [3 isomer predominates. Where the starting material is methyl naphthalene the final product is a mixture of a number of monoalkyl isomers.

The dielectric liquids of this invention are characterized by having a boiling range of about 260 to 360 C. at atmospheric pressure, corresponding to 110 to 175 C. at 3 millimeters Hg pressure, a pour point ranging from 0 C. to 40 C., and superior resistance to oxidation. Tests have indicated that the liquids of this invention possess sufficient oxidation stability to enable them to remain essentially unchanged after many years of use under normal operating conditions. The factthat these liquids are not at all decomposed by distillation at atmospheric pressure is a further indication of their oxidation stability.

The oxidation stability of the liquids of this invention enables transformers to operate satisfactorily at a higher operating temperature. The maintenance of a liquid temperature of C. is readily feasible. Thus, a trans- I former having incorporated therein the dielectric liquids of this invention will have a higher kva. rating than the same size transformer using conventional dielectric liquids.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. Therefore, I aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A transformer comprising a core piece, a primary winding on said core piece, a secondary winding on said core piece, a liquid-tight housing encasing said windings and core piece, and a liquid in said housing surrounding said windings and core piece, said liquid having a boiling point below C. at a pressure of 3 mm. of mercury and consisting essentially of a compound of the structure wherein R is selected from the group consisting of hydrogen and a methyl group and R is a tertiary alkyl group containing from 4 t0 8 carbon atoms.

2. A transformer as claimed in claim 1 wherein R is a tertiary butyl group.

3. A transformer as claimed-in claim 1 wherein R is a tertiary amyl group.

4. A transformer as claimed in claim 1 wherein R is a methyl group and R is a tertiary butyl group.

5. A transformer as claimed in claim 1 wherein R is a methyl group and R isa tertiary amyl group.

6. A transformer as claimed in claim 1 wherein R is a tertiary octyl group.

7. A transformer comprising a core piece, a plurality of windings positioned on said core piece, a liquid-tight container encasing said windings and core piece, a liquid in said container contacting said core piece and windings, said liquid having a boiling point below 150 C. at a pressure of 3 mm. of mercury and consisting essentially of a compound of the structure:

wherein R is selected from the group consisting of hydrogen and a methyl group and R is a tertiary alkyl group containing from 4 to 8 carbon atoms, and means for I wherein R is, selected from the group consisting of hy drogen and a methyl group and R is a tertiary alkyl wherein R is selected from the group consisting of hydrogen and a methyl group and R is a tertiary alkyl group containing 4 to 8 carbon atoms, said material having a boiling point below 150 C. at a pressure of 3 mm. of mercury.

References Cited in the file of this patent UNITED STATES PATENTS 2.245,721 Ross June 17, 1941 2,390,835 Hennion Dec. 11, 1945 2,436,110 Larsen Feb. 17, 1948 2,572,808 Jackson Oct. 23, 1951 

1. A TRANSFORMER COMPRISING A CORE PIECE, A PRIMARY WINDING ON SAID CORE PIECE, A SECONDARY WINDING ON SAID CORE PIECE, A LIQUID-TIGHT HOUSING ENCASING SAID WINDINGS AND CORE PIECE, AND A LIQUID IN SAID HOUSING SURROUNDING SAID WINDINGS AND CORE PIECE, SAID LIQUID HAVING A BOILING POINT BELOW 150*C. AT A PRESSURE OF 3 MM. OF MERCURY AND CONSISTING ESSENTIALLY OF A COMPOUND OF THE STRUCTURE.
 9. A DIELECTRIC MATERIAL HAVING HIGH RESISTANCE TO OXIDATION AND LOW POUR POINT CONSISTING ESSENTIALLY OF AN ISOMERIC MIXTURE OF COMPOUNDS OF THE STRUCTURE 